1. Field of the Invention
The present invention is related to a voltage regulator and related method, and more particularly, to a voltage regulator which provides sequentially and arbitrarily shaped regulated voltage and related method.
2. Description of the Prior Art
In electronic products, voltage regulators are usually disposed between a power supply circuit and a load circuit. The function of a voltage regulator is to provide a stable output voltage and a wide-ranged output current. When the load current suddenly changes, the output voltage can then be stabilized at its original level for providing efficient voltage conversion. For portable devices such as mobile phones, personal digital assistants (PDAs) and notebook computers, the voltage of the battery drops with time and is unable to maintain at a stable level. A low dropout (LDO) regulator can continuously provide a stable output voltage to the load circuit of an electronic device as long as the voltage difference between the input voltage provided by the battery and the estimated output voltage of the LDO regulator is larger than a dropout voltage.
Reference is made to FIG. 1 for a diagram illustrating a prior art LDO regulator 10. The LDO regulator 10 includes an error amplifier 110, a power device 120, a voltage-dividing circuit 130, and an output capacitor Co. The LDO regulator 10 is configured to convert an input voltage VIN into an output voltage VOUT for driving a load (represented by a resistor RL) through which a current IL flows. The voltage-dividing circuit 130, including resistors R1 and R2, is configured to generate a feedback voltage VFB corresponding to the output voltage VOUT by voltage-dividing the output voltage VOUT. The error amplifier 110 is configured to generate a control signal VSW by comparing the feedback voltage VFB with a reference voltage VREF. The output capacitor Co, coupled in parallel with the load RL, provides the load RL with current compensation when the load current IL suddenly changes, thereby improving the transient response of the output voltage VOUT. The power device 120 may be a P-channel metal oxide semiconductor (PMOS) switch having a gate for receiving the control signal VSW from the error amplifier 110, a source for receiving the input voltage VIN, and a drain for receiving the output voltage VOUT. When the feedback voltage VFB is smaller than the reference voltage VREF, the control signal VSW generated by the error amplifier 110 increases the output current of the power device 120; when the feedback voltage VFB is larger than the reference voltage VREF, the control signal VSW generated by the error amplifier 110 decreases the output current of the power device 120. Therefore, the LDO regulator can stabilize the output voltage VOUT at a predetermined value VOUT—NON. The relationship between the output voltage VOUT and the reference voltage VREF is depicted as follows:VOUT=(R1+R2)*VREF/R1 
where (R1+R2)/R1 has a constant value.
In a modern wireless transceiver, its receiver RX and transmitter TX operate alternatively, in which only one of the receiver RX and the transmitter TX is activated at a specific time. The transmitter TX is activated only during the transmitting bursts of communication packages, and is otherwise deactivated in order to reduce power consumption. The transmitter TX is required to provide output signal of unvarying characteristics (such as constant output power and phase) anytime during a transmitting burst. However, the circuit of the transmitter TX (such as a power amplifier) has a certain turn-on response time and a certain turn-off response time, both of which normally vary with temperature. In order to maintain unvarying signal characteristics, the time response of the transmitter needs to be compensated by, for instance, adjusting the bias voltage of the transmitter TX or the supply voltage of the receiver RX as the time elapses. In both cases, the bias voltage and the supply voltage are normally generated by the voltage regulator.
Reference is made to FIG. 2 for a diagram illustrating the operation of a prior art wireless transceiver. The waveforms depicted in FIG. 2 represent the bias voltage of the transmitter TX or the supply voltage of the receiver RX provided by the LDO regulator 10. The transmitting bursts of the transmitter TX are represented by BT1-BTn, while the receiving bursts of the receiver RX are represented by BR1-BRn. As previously stated, the turn-on response time and the turn-off response time of the transmitter TX and the receiver RX vary with temperature. Since the prior art LDO regulator 10 does not provide compensation, the prior art wireless transceiver may not be able to provide unvarying signal characteristics during the transmitting/receiving bursts of different communication packages.